This invention relates to methods of metallizing non-conductive substrates. The invention also relates to non-conductive substrates having a metallized surface. Particular applicability can be found in the optoelectronics industry in metallizing optical fibers and in the formation of hermetic optoelectronic device packages which include a metallized optical fiber.
Signal transmission using pulse sequences of light is becoming increasingly important in high-speed communications. Optical fibers have been a cornerstone in the infrastructure required for optical communications. The optical fibers are typically connected to optoelectronic components such as laser diodes, light emitting diodes (LEDs), photodetectors, modulators, and the like, in a device package. The resulting glass-to-metal connection between the optical fiber and package creates a hermetically sealed structure. Hermetic packages provide for containment and protection of the enclosed devices, which are typically sensitive to environmental conditions. In this regard, degradation in operation of optical and optoelectronic components may be caused by atmospheric contaminants such as humidity, dust, chemical vapors, and free ions. The optical input/output surfaces of the components in the package are especially susceptible to contamination while metallic surfaces of the package are susceptible to corrosion. Both of these effects can give rise to reliability problems. Hermetic sealing of the package to prevent contact with the outside atmosphere is thus desired.
To allow bonding of the optical fiber to an optoelectronic device package and formation of a hermetic seal, a metal structure is formed on the non-conductive, silica surface of the optical fiber. Several techniques for metallizing optical fibers are known in the art. For example, physical vapor deposition (PVD) techniques such as sputtering and evaporation, electroless plating, and a combination of electroless and electrolytic plating techniques have been proposed. Typical metal structures employed in fiber metallization include one or more nickel layer in combination with one or more additional metal layer such as gold. U.S. Pat. No. 6,251,252, for example, discloses formation of a first, electroless nickel layer on the optical fiber silica surface, a second, electrolytic nickel layer on the first nickel layer, and an electrolytic gold layer on the second nickel layer.
When nickel is used as the outer metal, solderability of the fiber becomes greatly reduced due to oxide formation at the nickel surface. The gold cap layer over the nickel is used as a result of nickel's propensity to oxidize, in an effort to maintain a solderable finish. The use of gold alone, however, is not completely satisfactory in eliminating solderability issues. In this regard, gold is a porous material and may not totally prevent oxidation of an underlying nickel surface, particularly when present as a very thin film. Depending on the gold thickness, these oxidized nickel areas may protrude above the surface of the gold. In addition, exposed nickel areas may remain even after gold formation due to coating uniformity issues, which areas are readily oxidized. Additional problems may arise as a result of the porosity of nickel and gold. In this regard, water vapor may pass from the atmosphere through the gold and nickel layers to the fiber clad. The water vapor can lead to microcrack formation in the fiber, resulting in optical loss and other reliability issues.
There is thus a continuing need in the art for improved methods of forming metallized fibers that overcome or conspicuously ameliorate one or more of the foregoing problems associated with the state of the art.